ACE is a dipeptidyl carboxypeptidase whose natural substrates include the oligopeptides angiotensin I and bradykinin. It has two isozymes, sACE and gACE, which have similar enzymatic activities but are expressed in a tissue-specific manner: gACE expression is restricted to developing male germ cells, whereas sACE is expressed in vascular endothelial cells, kidney tubular epithelial cells, intestinal brush border cells, monocytes and specific cell types in the brain. ACE-/- mice not only have lower blood pressure, but they also have abnormalities in kidney structure and function, and the male mice are sterile. By tissue-specific expression of transgenes encoding only one of the two isozymes of ACE, we have demonstrated that expression of sACE in the vascular endothelial cells is absolutely needed for blood pressure regulation; circulating sACE is insufficient for this function, although it can rectify the kidney problems of the Ace-/- mouse. In this application it is hypothesized that, in addition to its ability to produce angiotensin II, vascular endothelial cell-bound sACE has another activity to mediate outside-in signaling that is absolutely required for blood pressure regulation. To test this hypothesis, several transgenic sACE mutants will be expressed in Ace-/- mice using a vascular endothelial cell-specific transcriptional promoter. These transgenic studies will be complemented with tissue-specific and temporally regulated ablation of expression of the ACE gene. These studies will clearly delineate the relationships between specific physiological functions of ACE and its expression in specific tissues. The proposed molecular approach using a combination of transgenic and conditional knock-out techniques will lead to a better understanding of the basis of multiple functions of ACE and may lead to the development of clinical protocols for blocking a specific function of ACE without inhibiting others.